1. Field of the Invention
This invention generally relates to printed circuit board (PCB) design and, more particularly, to a PCB design permitting substrate flexibility, such as might be useful with a flexible electronic array.
2. Description of the Related Art
Many reliability problems have been associated with the use of flexible thin-film active-matrix arrays, when subjected to repetitive or extreme flexing. It is well-known that when subjecting the materials that are used to make the array to high stress/strain, that micro-cracks, delamination, and other defects appear. These defects result in the degradation of the film's performance, and eventually in its complete failure. In particular, conductive, brittle layers are especially prone to this problem. Even softer materials, such as aluminum, will exhibit degradation in their conductivity under repetitive flexing, before failure in the form of electrical opens. This problem is currently addressed by (a) confining the flexible array in a suitable enclosure, which allows limited flexing, so that extreme bending radius is not allowed, and flexing is confined to certain axes or directions of the array plane, or (b) laminating protective layers at the top/bottom to mitigate stresses in the active plane, which also limits flexibility.
FIG. 1 shows an exemplary layout of the gate and data lines in a typical active-matrix liquid-crystal display (AMLCD) array (prior art). A detail of 3×3 pixels is shown. Experimental data has shown that it is these long lines, with a typical width of 5-20 microns, thicknesses on the order of 1 micron, and a length in the 10-100 centimeter (cm) range (depending on array size and resolution), that are the main failure points under repetitive or tight radius bending.
FIGS. 2A and 2B depict the application of stress to a conventional straight line on a PCB (prior art). Shown in FIG. 2A is the line prior to the application of stress. When tensile stress is applied in the X-axis (FIG. 2B), this design is shown to suffer from crack formation in the metal film length, resulting in degraded conductivity, and eventually open-circuit (breakage) failure. At compressive stress along the X-axis, delamination is often seen, depending on the metal and substrate material.
It would be advantageous if conductive lines on a PCB could he designed to have greater resistance to flexing and stress.